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Align the places
Write ones below ones, tens below tens, and hundreds below hundreds. Start with ones. Addends are the numbers being added; the sum is the result.
Exchange ten for one larger unit
If the ones total 10 or more, exchange ten ones for one ten. Leave the remaining ones in the ones place. Add the new ten to the tens. Similarly, ten tens become one hundred.
Estimate, then check
Before calculating exactly, estimate the size. For 472 + 268, a result near 500 + 300 = 800 is reasonable. An estimate is not the exact answer, but a result of 140 signals a mistake.
Worked examples
472 + 268 = 740
Ones: 2 + 8 = 10, write 0 and carry 1 ten. Tens: 7 + 6 + 1 = 14, write 4 and carry 1 hundred. Hundreds: 4 + 2 + 1 = 7.
385 + 147 = 532
5 + 7 = 12; 8 + 4 + 1 = 13; 3 + 1 + 1 = 5. Check: 532 − 147 = 385.
Try it yourself
Your turn
Type the missing word, number, or sign. Spaces in large numbers are allowed. Try first, then open the explanation.
A library receives 186 picture books and 237 other books. Find the total, then check by subtraction.
See an example answer
186 + 237 = 423. Check: 423 − 237 = 186.
Check your knowledge
Choose one answer for each question. You can revise your answers and check again.
Answer key
- 579. 6 + 3 = 9; 2 + 5 = 7; 4 + 1 = 5.
- 824. Regroup one ten, then one hundred.
- One ten. We exchanged ten ones for one ten.
- 595 − 268 = 327. Subtract one addend from the sum to recover the other.
- 1 000. 5 + 5 = 10; 8 + 1 + 1 = 10; 6 + 3 + 1 = 10 hundreds.